Not only are these three words spelled similarly, they also have similar meanings. The main “basis” of these words is to guarantee something in some way, shape, or form. However, there are small intricacies other than that, so let’s take a look at these words individually.
This form is most commonly used when a person says or writes down a guarantee. I tried to think of an awesome memory hook, but all I got was “talk = Assure” The “a” in talk can help remind you of the “a” for “assure.” Let’s look at an example. “I assure you, everything will be alright.” In this case, the speaker is giving a verbal guarantee that everything will be fine. “The letter assured her that she would be compensated.” In this case, it is written. I view assure more like a “comforting” guarantee that someone would try to say to you.
Whereas assure is a verbal or written guarantee, ensure is when you do something to guarantee something happens. Ever hear of the drink Ensure? Basically it’s a health drink that claims you’ll be a healthier person if you drink it. Therefore, you’re DOING something that guarantees your health. Make sense? Let’s look at an example. “I put a deadbolt on my door to ensure no one will break in.” Because you installed a deadbolt, you believe your action guarantees your safety.
So assure is verbal or written, ensure is an action, and that leaves us with insure. Insure is the basis of insurance, which is a good way to remember this word. This guarantee is done though insurance, or some other form of financial or legal thing. An example is “I have insured my house against fires.” This would mean you are guaranteed a level of protection from the insurance company if you do have a fire.
Hope this helps, and good luck!